1. Field of the Invention
The present invention relates to biasing techniques in integrated circuit (IC) chips. More specifically, the present invention relates to a method and an apparatus for biasing a floating node in a system that supports proximity communication.
2. Related Art
Advances in semiconductor technology presently make it possible to integrate large-scale systems, including hundreds of millions of transistors, into a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip increases the speed at which such systems can operate, because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays. Moreover, integrating large-scale systems onto a single semiconductor chip significantly reduces production costs, because fewer semiconductor chips are required to perform a given computational task.
However, these semiconductor chips still need to communicate with each other, and unfortunately, these advances in semiconductor technology have not been matched by corresponding advances in inter-chip communication technology. Semiconductor chips are typically integrated onto a printed circuit board that contains multiple layers of signal lines for inter-chip communication. However, signal lines are typically 100 to 1000 times denser on a semiconductor chip than on a printed circuit board. Consequently, only a tiny fraction of the signal lines on a semiconductor chip can be routed across the printed circuit board to other chips. This problem is creating a bottleneck that is expected to worsen as semiconductor integration densities continue to increase.
One solution to the above problem is to replace the direct conductive coupling with direct chip-to-chip capacitive coupling, referred to as “Proximity Communication.” Proximity communication involves integrating arrays of capacitive transmitters and receivers onto semiconductor chips to facilitate inter-chip communication. If a first chip is situated face-to-face with a second chip so that transmitter pads on the first chip are capacitively coupled via a coupling capacitor with receiver pads on the second chip, it becomes possible to transmit electrical signals directly from the first chip to the second chip without having to route the electrical signal through intervening signal lines within a printed circuit board.
However, a capacitively coupled communication channel blocks the transmission of a DC component in the electrical signal. Moreover, the input node of the receiver amplifier, which is coupled to the receiver pad of the coupling capacitor, may not have a well-defined bias voltage, and is often floating. Consequently, in order to extract the DC level in the transmitted signal and to properly recover the transmitted information, the floating node which is coupled to the input of the receiver is typically biased to an appropriate DC voltage with external biasing circuits. This DC voltage sets a suitable DC operating point for the input of the receiver.
A number of biasing techniques have been proposed to set the DC bias voltage to a floating node. One existing technique refreshes the receiver inputs by periodically precharging/postcharging the receiving wires to a predetermined bias voltage. However, this technique can cause substantial power consumption. Moreover, attempts to make the periodical refreshing less frequent to reduce power consumption can cause further complexity in the biasing control mechanism.
Another existing biasing technique refreshes inputs by occasionally stopping the transmission of data across the coupling channels when necessary. Unfortunately, the process of stopping and resuming data communication can take a substantial amount of time. Furthermore, hiding the resulting delay can add additional design complexity.
Yet another existing biasing technique uses negative feedback from the output of a receiver to bias the input wire of the receiver. However, this technique adds complexity to the design and consumes a large amount of power.
Hence, what is needed is a method and an apparatus for biasing such floating nodes without the above-described problems.